The goal of these studies is to increase understanding of the pathogenesis of parasitic diseases and to improve their diagnostic, therapeutic and preventative measures. In addition to studies at NIH, clinical investigation of patients has been carried out in the endemic areas of Guatemala (onchocerciasis), India and Brazil (filariasis), Zaire and Jamaica (strongyloidiasis) Honduras (cysticercosis and leishmaniasis) and Panama (leishmaniasis). Such studies have clearly defined an association between HTLV-1 infection and strongyloidiasis, but there seems to be no similar relationship with HIV infections. Laboratory studies have permitted definition of the regulatory and pathogenetic mechanisms underlying the IgE, IgG4, and eosinophil responses to helminth infection, as well as the identification of molecules responsible for antigenic variation in giardiasis likely involved in protective immunity in bancroftian filariasis, onchocerciasis and strongyloidiasis. Therapeutic trials of ivermectin for bancroftian filariasis have shown it to be effective and safe in clearing microfilaremia and, because of its single-oral-dose mode of therapy, to be considerably more practical for use in control programs than the older antifilarial drug, diethylcarbamazine.